Two-photon or multiphoton absorption occurs through the simultaneous absorption of two or more photons via virtual states in an absorbing medium, with the former being more common. For a given chromophore, these absorption processes take place at wavelengths much longer than the cut-off wavelength of its linear (single-photon) absorption. In the case of two-photon absorption (2PA), two quanta of photons may be absorbed from a single light source (degenerate 2PA) or two sources of different wavelengths (non-degenerate 2PA). While multiphoton absorption processes have been theoretically described in 1931 and experimentally confirmed about 30 years later, this field remained dormant largely due to the lack of materials with sufficiently large two-photon sensitivity, quantified as two-photon cross-section (σ′2), which is usually expressed in the units of Göppert-Mayer (1 GM=1−50 cm4·s·photon−·molecule−1).
Then, in the mid-1990s, several new classes of chromophores exhibiting very large effective σ′2 values were reported. In conjunction with the increased availability of ultrafast high-intensity lasers, the renewed interest has not only sparked a flurry of activities in the preparation of novel dye molecules with enhanced σ′2 values, but also in advancing many previously conceived applications based on 2PA process in photonics and biophotonics, which are now enabled by these new chromophores. It is important to recognize the following useful features of the 2PA phenomenon based on the fact that 2PA scales nonlinearly with the squared intensity of the incident laser beam: (a) upconverted emission, whereby an incident light at lower frequency (energy) can be converted to an output light at higher frequency, for instance, near infrared (NIR) to ultraviolet (UV) upconversion; (b) deeper penetration of incident NIR light (into tissue samples, for example) than UV light, which may be hazardous with prolonged exposure; (c) highly localized excitation, as compared with one-photon processes, allowing for precise spatial control of in situ photochemical or photophysical events in the absorbing medium, thereby minimizing undesirable activities, such as, photodegradation or photobleaching; and (d) fluorescence, when properly manipulated, that would allow for information/signal feedback or amplification in conjunction with other possible, built-in effects, such as, surface plasmonic enhancement.
It is anticipated that further ingenious utilization of these basic characteristics will lead to practical applications other than the ones that have already emerged in such diverse areas as bio-medical fluorescence imaging, data storage, protection against laser damage, microfabrication of microelectromechanical systems (MEMS), photodynamic therapy, etc. In the past decade or so, significant advances have been made in the fundamental understanding of general structure-property relationship that has led to the design and synthesis of two-photon absorbers with very large cross-section values. Although further enhancement of 2PA cross-section is still possible, as suggested by a number of theoretical studies, for certain applications the two-photon-property requirement has essentially been met by the state-of-art chromophores. Because of the possible property-processing/fabrication trade-off, the secondary properties, e.g., thermal and mechanical properties, which are important to material processing into various useful forms (films, coatings, fibers, windows, etc.) and configurations, should be addressed. For the aforementioned solid forms, polymers can offer many advantages, such as the flexibility in fine-tuning the material properties and the availability of many processing options.
Accordingly, it is an object of the present invention to provide two-photon absorbing (2PA) chromophores with molecular features that are amenable to hyper-branching polymerization or network-forming polymerization with suitable co-monomers containing reactive functions, such as organic isocyanate, carboxylic acids, acid chlorides, anhydride-acid chloride, anhydride-carboxylic acid, as well as silicon-based co-monomers, such as tetra-alkoxy silane Si(OR)4, commonly used in sol-gel processing of organically modified silicate glasses. The 2PA chromophores could also be further converted to thermosettable resins, including, but not limited to, acrylate and vinyldimethylsiloxane resins via reaction of alcohol groups with acrylic acid chloride or reaction of alcohol with vinyldimethylsilyl chloride, respectively. These thermosettable resins could be obtained with or without co-monomers via conventional free-radical polymerization or catalytic hydrosilylation processes.
Specifically, the main objective is to provide new compositions of matter, and methods of preparation thereof, for 2PA-active chromophores with a structural motif in which a 1,3,5-triazine core is triply connected to tertiary amino endgroups via 9,9-dialkyfluorenyl bridges. Another objective is to provide the methods for the preparation of these compounds, the compositions, and useful derivatives, which are obvious to those skilled in the art of thermosetting polymers.
Other objects and advantages of the invention will be set forth, in part, in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.